1. Field of the Invention
This invention relates to a novel method for forming coating films by coating a powdery composition of a thermoplastic resin having polymerizable unsaturated bonds on an article to be coated, heat-melting the powdery resinous composition and curing the molten film under application of ionizing radiations or ultraviolet rays.
2. Description of the Prior Art
As one of conventional methods for forming coating films, there is known a method comprising coating a powdery resinous composition formed by incorporating a curing agent, a pigment and other additives into a powdery resin, on an article to be coated according to a customary coating technique, for example, electrostatic coating or fluidized bed coating, and heating the coated composition to effect melting and curing simultaneously. This known method is advantageous in that as the coating composition is free of a solvent, and air pollution owing to evaporation of a solvent is not caused to occur. However, since the melting and curing steps are simultaneously conducted by heating, this method is defective in that since the curing reaction partially proceeds at the melting stage and bubbles are formed by the condensation reaction caused at the curing stage, it is difficult to obtain coatings having good surface appearance.
As another known method for forming coating films, there can be mentioned a method in which ionizing radiations or ultraviolet rays are applied to a coated film including a resin having polymerizable unsaturated groups to cause a chemical polymerization reaction and effect curing. Most of paints used in this method are formed by dissolving a resin having polymerizable double bonds into a polymerizable monomer such as styrene and methyl methacrylate. When these paint compositions are used for formation of coating film, the monomer used as the solvent is evaporated at the curing step under application of ionizing radiations or ultraviolet rays. Therefore, this method is defective not only in that economical disadvantages are brought about by the evaporation loss of the monomer but also in that the evaporated monomer emits an offensive odor and causes air pollution. Further, when compositions of resins having polymerizable unsaturated bonds are cross-linked and cured under application of ionizing radiations or ultraviolet rays, it frequently happens that abrupt shrinkage is caused to occur at the cross-linking step. This shrinkage phenomenon causes formation of strains and distortions in the cured coating, resulting in insufficient impact resistance, reduction of adhesion of the coating to a substrate and degradation of flexibility of the coating.
It is therefore a primary object of this invention to provide a method for forming coating films, which can overcome the foregoing defects and difficulties involved in the conventional coating methods.
Another object of this invention is to provide a method for forming coating films which can give coating films having excellent surface appearance and being excellent in such properties as the impact resistance, the adhesion to a substrate, the flexibility, the chemical resistance and the like.
Still another object of this invention is to provide a method for forming coating films according to which coating films having excellent surface appearance can easily be prepared by application of ionizing radiations or ultraviolet rays without causing air pollution or the like.